Anodic protection of kraft digesters



Nov. 21, 1961 w. A. MUELLER ETAL 3,009,365

ANQDIC PROTECTION OF KRAFT DIGESTERS Filed June 2. 195a 4 Sheets-Sheet 1 7 llllllllllz &

REF. j ELECTRODE INVENTORS WALTER A.MUELLER THOMAS R.B.WATSON p n/n/lfy ATTORNEYS.

Nov. 21, 1961 w. A. MUELLER ET AL 3,009,855

ANODIC PROTECTION OF KRAFT DIGESTERS Filed June 2, 1958 4 Sheets-Sheet 2 20 RECORDER INPUT FROM REFERENCE AMPLIFIER ALARM ELECTRODE 1 HIGH VOLTAGE RECTIFIER D. C. CURREN T T E TO CATHODES [M R LOW VOLTAGE RECTIFIER E FIG.

INVENTORS WALTER A. MUELLER ATTORNEYS.

Nov. 21, 1961 w. A. MUELLER ET AL 3,009,865

ANODIC PROTECTION OF KRAFTDIGESTERS Filed June 2, 1958 4 Sheets-Sheet 3 IOOAMP.

TO-LIQUOR POTENTIAL (VOLT) PASSIVE RANGE I -o.9 u o m 2 Lu n: Lu Lu 9 E Q-LO Q l IQ o I00 I20 :40 I60 TIME FROM START OF FILL IN MINUTES INVENTORS WALTER A. MUELLER THOMAS R.B. WA TSc/v ATTORNEYS.

NOV. 21, 1961 w A, MUELLER ET L 3,009,865

ANODIC PROTECTION OF KRAFT DIGESTERS Filed June 2, 1958 4 Sheets-Sheet 4 s E s 2 k Lu 5 g -0.8 lg a 3 E I 8 o 9 Lu g 5 a2 Lu 3 2 9 l- Q v l J S) 0 2O 4O 6O 80 I00 I20 I40 TIME FROM START OF FILL IN MINUTES /N\{ENTOR$ WALTER A.MuELLER THOMAS R. B. WATso/v BYMv ATTORNEYS E'- v a l.) Ellie 1 3,009,865 ANonrc PROTECTION on KRAFT nionsrnns Walter Adolf Mueller, 12 Mimosa Ave, Dorval, Quebec,

Canada, and Thomas Ramsay Bertin Watson, 15 Hudson Drive, Toronto, @ntario, Canada Filed June 2, 1958, Ser. No. 739,148 13 Claims. (Cl. 2tl4l47) This invention relates to a method of inhibiting the corrosion of pulp digesters. More particularly the invention is directed to a method of inhibiting the corrosion of mild steel pulp digester-s under the action of alkaline liquors in the sulphate pulping process.

In the kraft process for making pulp or paper, the logs are made into chips and charged into a digester. The digester is also charged partly with white liquor which is a newly made up solution of sodium hydroxide and sodium sulphide and partly with black liquor which is the residual solution left over from a previous digestion. The black liquor has a much lower concentration of sodium hydroxide than the white liquor and is often charged separately from the white liquor. The whole vessel is heated either by external circulating heaters, or by direct steam injection, and is brought up to the desired digestion temperature and pressure. The digestion takes about three hours from charging to blowdown or emptying, by which time the lignin is dissolved and the cellulose fibres freed. Considerable corrosion of the mild steel digesters is experienced.

Certain metals including steel have two electrochemical states which are manifestations of their surface condition. These states are known as the active and passive states and are detected and diiferentiated practically by the potential of the metal with reference to its environment. Metal in the active condition corrodes very readily while metal in the passive state tends to be quite corrosion resistant. Thus steel in its usual active form will have a potential more basic or electronegative than will steel which has been passivated by treatment with an oxidizing agent such as strong nitric acid. This potential is measured between the steel and a half cell electrode maintained in the electrolyte which is in contact with the steel. In the same way stainless steel owes its inherent corrosion resistance to a passive film, but if the passivity is spoiled through exposure to a reducing condition, the activated stainless steel will corrode as readily as ordinary steel and there will be a potential between the two in such a direction as to make the passive metal positive with respect to the active metal.

Some test work has been done on cathodic protection of mild steel digesters in which anodes are installed in the digester and a current is caused to flow between the digester and the anode. The entire digester shell is, however, maintained by this procedure in the active state although corrosion is reduced. A number of disadvantages have been found in this type of protection, for example, (a) the large amount of current required to maintain the total digester in the state of cathodic protection for the entire digestion period and (b) corrosion is only reduced, not prevented.

It has been discovered that mild steel in a mixture of black and white liquors used in the digestion of chips in the kraft process is in a borderline environment between active and passive states and is liable to swing from one condition to the other. This discovery was the result of numerous laboratory experiments which covered the conditions of the varied liquor composition of alkaline pulping digestions. The walls of the digester are naturally left in a passive condition after a digestion by reason of the conversion of white liquor to black liquor. However the incoming white liquor of anew run im- 3,009,865 Patented Nov. 21, 1961 mediately activates these surfaces, which stay active during the filling period and thereafter for, about two hours, after which the rising temperature and decreasing con centration of sodium hydroxide change the conditions in mavour of a sudden swing from the active to the passive condition. The digester usually remains in a passive state for the remainder of the digestion, but some active spots may remain so that some unnecessary corrosion may nevertheless continue.

Following the above discovery it has further been found that a much superior method of inhibiting the corrosion of pulp digesters is by the use of anodic protection instead of the previously used cathodic protection. In the passive range produced by the anodic protection, the potential of the digester steel is shifted to the positive side of the borderline potential between the active and passive ranges into the passive range and preferably sufllciently far from this borderline that areas which receive only a minor fraction of the average current density are still in passive state and well protected. Tests have revealed that mild steel immersed in kraft liquor usually displays a passive state at a potential of 0.5 to O.9 volt with reference to a saturated calomel electrode. However, -O.9 volt is on the borderline between the active and passive states and for practical purposes it has been found that it is desirable to maintain the potential of the metal surface in the range 0.6 to O.75 with reference to a saturated calomel electrode volt so that areas of the digester wall which may be below the average level still remain above the borderline potential and are thus also maintained passive and corrosion resistant. Potentials in the desirable range of 0.6 to 0.7 volt are rarely reached even during the later stages of a digestion in an unprotected digester.

As stated above, during the digestion there is normally a change in the conditions in the digester owing to the changing composition of the liquor, and a resultant change in the surface condition of the digester walls from the active to passive state. Anodic protection applied in accordance with this invention is capable of being used to prevent this normal development of the active state and to maintain the walls of the digester throughout the digestion at potentials sufliciently far removed from the borderline that corrosion is almost entirely eliminated. The current required for maintaining the required potential varies according to the nature of the digestion being carried out, the progress of the digestion, the size of the digester and probably also the composition of the steel of which the digester is made. For simplicity and for greater freedom from corrosion, it is convenient to use a somewhat greater current than that which will just maintain the required potential, and to use some current throughout the whole course of the digestion, although once the above noted change in the conditions has taken place, the current can be considerably reduced.

When a surface changes from the active to the passive state there is a sudden shift in potential. Thus, if an active area exists in an otherwise passive digester it will tend to have a significant influence on the average potential of the digester as indicated by the reference electrode since there is a substantial potential difference between the active and passive states. The current requirements can be reduced without changing liquor composition or type of steel by such methods as locally promoting the formation of calcium carbonate scale or using stainless steel in various forms to protect areas which receive a lower-than-average current density. In addition, the black liquor used for make up can be oxidized before making the mixture since oxidized black liquor has the effect of reducing both the quantity of current required and the length of the period for which it is required. Furthermore the digester may be heated by direct steaming instead of by indirect steaming.

The average current density required for satisfactory protection of mild steel digesters is usually not more than 15 amperes per square meter (1.4 am'peres per square foot) of surface exposed to the liquor, and under normal plant conditions, is more likely to be to amperes per square meter or even less depending on the above mentioned factors. Thus an 88 ton capacity digester, under normal plant conditions, would be expected to require about 500 amperes for complete protection.

The equipment required for the anodic protection of pulp digesters comprising one or more cathodes installed preferably in a vertical position in a digester, and preferably either centrally located or spaced equidistantly around the inside periphery of the shell, sufiicient supports for the cathodes, and insulated fittings to fasten the cathodes to the supports and/or the supports to the digester Walls. The material of construction of the cathode is not critical but steel has been found to be satisfactory and is preferred for economic reasons. The cathode assembly may be rigid or may be articulated or flexible and suspended only from the top. A source of direct current of variable voltage and amperage is also required to feed the current to the cathode, and one or more reference electrodes are required to determine the electrochemical state of the digester shell. The potential indicated by the reference electrode shows whether the digester shell is active or passive and can be designed either to activate an alarm, adjust the current automatically, or indicate the electro-chemical condition of the shell on a chart, or to do more than one of these operations. The reference electrode must be capable of continuously measuring the potential difference between the steel and the liquor for the proper application of anodic protection. For this purpose a saturated calomel reference electrode with a closed stem is preferred since it is capable of being used continuously for long periods without attention. Since the holding current used in the later stages of the digestion is much lower than that required earlier in the digestion, the source of direct current may be arranged as two units, one for high current operation and the other for low current operation, thus simplifying control. Normally these will be rectifier units supplied with alternating current.

In the drawings which illustrate the invention, FIG- URE l is an elevation showing a cathode assembly installed in the digester and FIGURE 2 illustrate the details of one of the flange connections in the cathode assembly. FIGURE 3 is a diagrammatic representation of one embodiment of the electrical apparatus to be used in association with a digester fitted with such a cathode assembly.

The equipment illustrated in FIGURE 1 comprises a mild steel digester shell 1 which contains a two-inch diameter standard steel pipe 2 as a cathode. The cathode, the length of which is approximately equal to one half of the total height of the digester, is centrally located on the longitudinal axis of the digester such that the upper end of the electrode is located at approximately the liquor level of the full digester. It is supported at top and bottom by horizontal assemblies consisting of pipes 3 fastened by flange joints 5 to the cathode 2 and pipes 6 which are fastened to pipes 3 by flange joints 5 and which are welded into holes in the digester wall. To ensure the rigidity of the cathode, inclined assemblies are also included consisting of pipes 4, which are welded to pipes 3 and fastened by flange joints 5 to pipes 8 which are welded to the interior surface of the digester. The flange joints 5 are insulated by means of asbestos centre gaskets with polytetrafluorethylene spacers and bolt sleeves. Direct current is supplied to the cathode by means of a conduction cable 7 which is carried inside a horizontal section 3 of the support assembly. The reference electrode, which is preferably a saturated calomel electrode with a closed stem, is located at the critical area near the liquor level in the digester.

in FIGURE 2, which illustrates the details of one of the insulated flange connections for the cathode, flanges It) are rigidly afiixed to pipe support 3 and cathode 2. A gasket 11, fabricated from asbestos encased by polytetraiiuorethylene, is inserted between the flanges to insulate them from each other. The flanges it} are fastened together by means of bolts 12 which are insulated from the flanges by polytetrafiuorethylene bolt sleeves 13 and polytetrafiuorethylene washers 14.

In FIGURE 3, which illustrates the electrical apparatus, two rectifiers, a high voltage rectifier l6 and a low voltage rectifier 17, feed the current to the digester system of FIGURE 1. A timer 1% is actuated by the blow valve or by the control valve on the incoming white liquor and is connected to the high voltage rectifier. The high voltage rectifier is controlled to a constant voltage in the order of four volts until the design current is reached after which it is controlled by current. This design current may be as high as 1,500 amperes per digester but under normal conditions is estimated to about 500 amperes. The current is controlled by the requirements as indicated by the reference electrode in the digester. The high current remains on for a predetermined period of time, say for 50 minutes, after which the timer automatically switches to the low voltage rectifier which controls the current at a holding value of say 200 amperes as determined by the reference electrode system. This lower current is maintained for the remainder of the di estion period and is turned off by the timer just before the blow-down.

An amplifier 19 amplifies the indications of the reference electrode and the output of the amplifier is fed to a recording millivolt meter 20 and to rectifier control circuits so that the current in either rectifier may be automatically increased by the system in the event that the digester tends towards the active state as indicated by the reference electrode. The current adjustments are made in quite large steps as determined by transformer taps so that the digester is always receiving substantially more current than the critical value. In the event that the reference electrode system adjusts the current in any rectifier to the maximum obtainable while still not maintaining passivity, or if the digester otherwise goes active, for instance through failure of the electrical equipment, an alarm 21 is caused to operate.

A series of tests were made on an 88 ton capacity digester into which had been installed an anodic protection assembly of the type described above. The tests were carried out on soft wood chips using a digestion liquor containing 22 to 43% of black liquor with the white liquor. The white liquor used was a solution containing 78 to grame/litre efiective Na O. Normal white liquor compositions are from 35 to 45 grams/litre Na S and 70 to 110 grams/litre NaOH.

For the purpose of the tests three reference electrodes were installed in the digester at locations Where the current supply was potentially insutficient. The bottom electrode was installed about three feet above the top of the bottom cone, the centre electrode was installed behind the stainless steel pump screen about half way up the digester, and the third one was installed near the top of the liquor level in the full digester, that is, about 14 feet from the top of the shell. The tests were carried out using both indirect steaming and direct steaming to heat the digester.

FIGURE 4 shows the typical results obtained from a test made while heating the digester by indirect steam. These curves indicate that 700 amperes for the first 55 minutes of the run were ample to maintain a passive state. The declining potential after the current was reduced indicates that amperes was a minimum holding current.

FIGURE 5 shows results obtained from a test carried out while the digester was being heated by direct steam. These results indicate that an initial current of 400 amperes and a holding current of 50 amperes were successful in maintaining the digester in a passive state.

FIGURES 4 and 5 also indicate that without the application of anodic protection the steel of the shell remains in the active state for two hours during which time it is Subject to severe corrosion. The shift in the potential-time curve after the two hour period indicates that corrosion had been markedly reduced. Nevertheless, the potential was not shifted to the preferred range.

As a result of the test runs considered above and other similar tests the average current density which was required for protection was found to be approximately 5 amperes per square meter during a directly steamed digestion at 32% black liquor. During indirectly steamed digestion, l amperes per square meter were required at 37% black liquor and 15 amperes per square meter at 27% black liquor. These current density requirements were unusually high due to the fact that the black to white liquor ratio was very low (a normal industrial alkaline pulping digester is charged With 50-60% black liquor) and the digester shell was not covered with its usual coating of calcium carbonate scale. In view of the above factors it has been estimated that an average current requirement will be approximately amperes per square meter.

Anodic protection has exhibited a number of advantages over the presently used cathodic protection. For instance previously cathodically protected areas when raised above the liquor level will corrode more rapidly than continuously unprotected areas and when the potential would naturally shift to the passive range the cathodic protection still keeps it in the active condition. Thus cathodic protection activates any area which would normally be passive at the end of the digestion cycle, and shielded areas such as the places behind screens, etc. which usually remain passive under normal operation only succeed in having their natural passivity spoiled by cathodic protection. Anodic protection also has the advantage that the cathodes used are not electrolytically consumed since the current density is sufliciently high on the cathodes to give them cathodic protection while the shell as a whole is getting anodic protection. This allows plain steel to be used for the cathode.

What we claim as our invention is:

1. A method of inhibiting the corrosion of mild steel pulp digesters under the action of alkaline liquors containing 22 to 60 percent black liquor in the sulphate pulping process which comprises providing the digester with a cathode electrically insulated from the walls of the digester, and supplying while the corrosive liquors are in contact with the digester, an electric current flowing between the cathode and the walls of the digester as anode of up to 15 amperes per square meter of digester shell exposed to the liquor, the magnitude of the current being such that the average potential of the walls of the digester is maintained in the range of from about 0.9 to about 0.5 volt with reference to a saturated calomel electrode.

2. A method of inhibiting the corrosion of mild steel pulp digesters under the action of alkaline liquors containing 35 to 60 percent black liquor in the sulphate pulping process which comprises providing the digester with a cathode electrically insulated from the walls of the digester, and supplying while the corrosive liquors are in contact with the digester, an electric current flowing between the cathode and the walls of the digester as anode of up to 15 amperes per square meter of digester shell exposed to the liquor, the magnitude of the current being such that the average potential of the walls of the digester is maintained in the range of from -0.8 to 0.5 volt with reference to a saturated calomel electrode during the first half of the digestion.

3. A method of inhibiting the corrosition of mild steel pulp digesters under the action of alkaline liquors containing 35 to 60 percent black liquor in the sulphate pulping process and heated by direct sterning which comprises providing the digester with a cathode electrically insulated from the walls of the digester, and supplying while the corrosive liquors are in contact with the digester, an electric current flowing between the cathode and the walls of the digester as anode of 5 amperes per square meter of digester shell exposed to the liquor, the magnitude of the current being such that the average potential of the walls of the digester is maintained in the range of from about 0.9 to about 0.5 volt with reference to a saturated calomel electrode.

4. A method of inhibiting the corrosion of mild steel pulp digesters under the action of alkaline liquors containing 35 to 60 percent black liquor in the sulphate pulping process and heated by indirect steaming which comprises providing the digester with a cathode electrically insulated from the walls of the digester, and supplying while the corrosive liquors are in contact with the digester, an electric current flowing between the cathode and the walls of the digester as anode of 15-10 amperes per square meter of digester shell exposed to the liquor, the magnitude of the current being such that the average potential of the walls of the digester is maintained in the range of from about 0.9 to about -'-0.5 volt with reference to a saturated calomel electrode.

5. A method of inhibiting the corrosition of mild steel pulp digesters under the action of alkaline liquors containing 35 to 60 percent black liquor in the sulphate pulping process and heated by direct steaming which comprises providing the digester with a cathode electrically insulated from the Walls of the digester, and supplying while the corrosive liquors are in contact with the digester, an electric current flowing between the cathode and the walls of the digester as anode of 5 amperes per square meter of digester shell exposed to the liquor, the magnitude of the current being such that the average potential of the walls of the digester is maintained in the range of from 0.8 to 0.5 volt with reference to -a saturated calomel electrode during the first half of the digestion.

6. A method of inhibiting the corrosition of mild steel pulp digesters under the action of alkaline liquors containing 35 to 60 percent black liquor in the sulphate pulping process and heated by indirect steaming which comprises providing the digester with a cathode electrically insulated from the walls of the digester, and supplying while the corrosive liquors are in contact with the digester, an electric current flowing between the cathode and the walls of the digester as anode of 15-10 amperes per square meter of digester shell exposed to the liquor, the magnitude of the current being such that the average potential of the walls of the digester is maintained in the range of from 0.8 to 0.5 volt with reference toa saturated calomel electrode during the first half of the digestion.

7. A method of inhibiting the corrosion of mild steel pulp digesters under the action of alkaline liquors containing 35 to 60 percent black liquor in the sulphate pulping process when treating soft wood which comprises providing the digester with a cathode electrically insulated from the walls of the digester, and supplying while the corrosive liquors are in contact with the digester, an electric current flowing between the cathode and the walls of the digester as anode of up to 15 amperes per square meter of digester shell exposed to the liquor, the magnitude of the current being such that the average potential of the walls of the digester is maintained in the range of from about 0.9 to about 0.5 volt with reference to a saturated calomel electrode.

8. A method of inhibiting the corrosion of mild steel pulp digesters under the action of alkaline liquors containing 22 to 60 percent oxidized black liquor in the sulphate pulping process which comprises providing the digester with a cathode electrically insulated from the walls of the digester, and supplying While the corrosive liquors are in contact with the digester, an electric current flowing between the cathode and the walls of the digester as anode of amperes per square meter of digester shell exposed to the liquor, the magnitude of the current being such that the average potential of the walls of the digester is maintained in the range of from about 0.9 to about 0.5 volt with reference to a saturated calomel electrode.

9. A method of inhibiting the corrosition of mild steel pulp diges-ters under the action of alkaline liquors containing 22 to 60 percent oxidized black liquor in the sulphate pulping process which comprises providing the digester with a cathode electrically insulated from the walls of the digester, and supplying while the corrosive liquors are in contact with the digester, an electric current flowing between the cathode and the walls of the digester as anode of 5 amperes per square meter of digester shell exposed to the liquor, the magnitude of the current being such that the average potential of the walls of the digester is maintained in the range of from 0.8 to --0.5 volt with reference to a saturated calomel electrode during the first half of the digestion.

10. A method of inhibiting the corrosition of mild steel pulp digesters under the action of alkaline liquors containing 22 to 60 percent oxidized black liquor in the sulphate pulping process when treating soft wood which comprises providing the digester with a cathode electrically insulated from the walls of the digester, and supplying while the corrosive liquors are in contact with the digester, an electric current flowing between the cathode and the walls of the digester as anode of up to 15 amperes per square meter of digester shell exposed to the liquor, the magnitude of the current being such that the average potential of the walls of the digester is maintained in the range of from about 0.9 to about --0.5 vol-t with reference to a saturated calomel electrode.

11. A method of inhibiting the corrosion of mild steel pulp digesters under the action of alkaline liquors containing white liquor of 30-45 grams/litre Na S and 70-110 grams/litre NaOH, and 35 to 60 percent black liquor in the sulphate pulping process which comprises providing the digester with a cathode electrically insulated from the Walls of the digester, and supplying while the corrosive liquors are in contact with the digester, an

electric current flowing between the cathode and the walls of the digester as anode of up to 15 amperes per square meter of digester shell exposed to the liquor, the magnitude of the current being such that the average potential of the Walls of the digester is maintained in the range of from about 0.9 to about -0.5 volt with reference to a saturated calomel electrode.

12. A method of inhibiting the corrosion of an 88 ton capacity mild steel pulp digester under the action of alkaline liquors containing 22 to 43 percent black liquor in the sulphate pulping process and heated by indirect steaming which comprises providing the digester with a cathode electrically insulated from the walls of the digester, and supplying while the corrosive liquors are in contact with the digester an electric current flowing between the cathode and the Walls of the digester as anode of 600 to 800 amperes for the first 40 to minutes of the digestion and a current of to 200 amperes for the remainder of the digestion.

13. A method of inhibiting the corrosion of an 88 ton capacity mild steel pulp digester under the action of alkaline liquors containing 22 to 43 percent black liquor in the sulphate pulping process and heated by direct steaming which comprises providing the digester with a cathode electrically insulated from the walls of the digester, and supplying 'while the corrosive liquors are in contact with the digester an electric current flowing between the cathode and the walls of the digester as anode of 300-500 amperes for the first 40 to 50 minutes of the digestion and a current of 30 to 50 amperes for the remainder of the digestion.

References Cited in the file of this patent UNITED STATES PATENTS 1,825,477 Reichart Sept. 29, 1931 2,576,680 Guitton Nov. 27, 1951 ,803,540 Durant et 'al. Aug. 20, 1957 2,905,604 Kennedy et al. Sept. 22, 1959 OTHER REFERENCES Evans: Metallic Corrosion Passivity and Protection, 1948, pp. 12, 55, 

1. A METHOD OF INHIBITING THE CORROSION OF MILD STEEL PULP DIGESTERS UNDER THE ACTION OF ALKALINE LIQUORS CONTAINING 22 TO 60 PERCENT BLACK LIQUOR IN THE SULPHATE PULPING PROCESS WHICH COMPRISES PROVIDING THE DIGESTER WITH A CATHODE ELECTRICALLY INSULATED FROM THE WALLS OF THE DIGESTER, AND SUPPLYING WHILE THE CORROSIVE LIQUORS ARE IN CONTACT WITH THE DIGESTER, AN ELECTGRIC CURRENT FLOWING BETWEEN THE CATHODE AND THE WALLS OF THE DIGESTER AS ANODE OF UP TO 15 AMPERES PER SQUARE METER OF DIGESTER SHELL EXPOSED TO THE LIQUOR, THE MAGNITUDE OF THE CURRENT BEING SUCH THAT THE AVERAGE POTENTIAL OF THE WALLS OF THE DIGESTER 